AM2634-Q1: AM263P4

Hello Rapeti,

The AM263x/AM263Px ADC_VREF has a requirement of 1.8V +/- 5%. This is internally scaled to the 3.2V input voltage range (hence the reason that the 3.2V number is used in the ADC TINA model.

I would like to clarify your test setup. It appears that you are using the device's own ADCs to measure its own VREF voltage. Is that correct?

If so, I would like to point out that this is not a valid test configuration. The ADCs internally utilize the ADC reference voltage (scaled up from 1.8V) to charge and discharge internal components during the sample/hold and compare/convert processes. By externally shorting the reference source voltage to the ADC input, you are creating a feedback loop that can lead to erratic and inaccurate results. This configuration is not tested or recommended by TI.

Using any external apparatus to test the VREF voltage, may still introduce noise, impedance, and instability into the reference circuit, which can affect the ADC readings (even if connected to sources other than its own reference voltage).

To ensure accurate results, we recommend isolating your VREF voltage source/testing from your ADC input circuitry source/testing to prevent unwanted analog feedback loops between the ADC input and the ADC voltage reference.

Fleenor said:

I would like to clarify your test setup. It appears that you are using the device's own ADCs to measure its own VREF voltage. Is that correct?

No. Actually I meant to say that i have enabled the adc internal Vref voltage using the REFBUF0 register(by ensuring 111 in it ) and observed the Vref voltage at the switch s2 VrefHi G01 pin which i got as 1.79v. Now i applied different voltages at ADC0 Channel0 using regualted power supply and observed the counts mentioned in the above table.

Fleenor said:

The AM263x/AM263Px ADC_VREF has a requirement of 1.8V +/- 5%. This is internally scaled to the 3.2V input voltage range (hence the reason that the 3.2V number is used in the ADC TINA model.

So internal Vref voltage which is 1.8v is scaled to 3.2v?

How can i verify this and is there any document, where i can found the Vref scaled voltage of 3.2 value.

Hey Rapeti,

Understood, thanks for the clarification. This fluctuation is expected due to the reasoning provided in the previous response. The VREF is used by the ADC during conversion, it could either be sourcing current or sinking current based on the ADC operation. The ADC conversion load will differ based on a variety of factors including the last sample voltage which is still present on the sample capacitor at the start of the next conversion.

What is meant by "counts" in this context? What are the voltages you are applying ADC_IN0?

The scaling factor of 32/18 is represented Input Conversion Range function of the Datasheet - Electrical Specifications - ADC section.

32/18 × V_REFHI

I will make sure some text is added to the TRM chapter to make sure this is absolutely clear.

Best Regards,

Zackary Fleenor

Thanks for the reply,

Fleenor said:

What is meant by "counts" in this context? What are the voltages you are applying ADC_IN0?

These are the input voltage applied to ADC0 AIN0 pin and here "counts" is the corresponding digital output.

Can i know how the scaling of Vref is done from 1.8v to 3.2v?

Hey Rapeti,

The traditional SAR ADC operation usually requires your maximum input voltage to be less than or equal to the VREF voltage, anything above this level is saturated at max code output. The AM26x devices use a novel internal design to allow input voltages well above the 1.8V VREF.

There are two scenarios in which this scaling could be accomplished.

1) Scale VREF up from 1.8V

2) Scale ADC Input Voltage down to 1.8V

The AM26x ADCs use the second option with a unique implementation of the input sampling network to enable full scale input with a 1.8V VREF.

The exact details of this implementation are internal to TI.

Best Regards,

Zackary Fleenor

Hi Fleenor,

So ADC Vref voltage is 1.8v and input voltage above 1.8v are scaled down, with reference to 1.8v Vref right?

And till what range the input voltage can be applied?

Could you please mention a formula to calculate the digital output of the adc.

Regards,

Sravanthi R.